Why might you use a Project instead of a single standalone conversation?

The reason is continuity of context. In a standalone chat, everything Claude knows about your situation is what you typed into that chat, so work that runs over weeks means re-pasting the same specification, the same schema or the same style guide every time you open a new one — slow, and a reliable way for three slightly different versions of the same document to end up in circulation.

A Project moves that material up a level. Documents, code and notes go into the Project's knowledge base, project instructions record the standing brief — the tone to use, the role to answer from, the conventions to follow — and every chat you start in the Project begins with both already attached. You describe the background once, and the tenth conversation starts as well-informed as the first.

That is a workflow benefit rather than a capability unlock. Standalone chats still produce Artifacts and still accept file uploads. What a Project changes is how much of the setup you have to repeat, which is exactly what makes it worth the trouble for ongoing work.